Sensitive Antarctic species including giant sea spiders, huge submarine relatives of the woodlouse and sea gooseberries the size of footballs could perish as Southern ocean waters warm by 2-3C (3.6-5.4F) in the next century, scientists warned yesterday.

"We are talking about thousands of species," Lloyd Peck, of the British Antarctic Survey, told the British Association for the Advancement of Science festival in Leicester. The bizarre sea bottom-dwelling creatures would be unable to function as temperatures warmed and would be threatened by competition from invaders from warmer waters.

Professor Peck and other researchers had looked at 750 species of amphipod - relatives of sand fleas - and found that all are at risk from a sea change. Arthropods and bivalves, important sources of food for fish and higher sea creatures, are also threatened. Ultimately, massive deaths low in the food chain could affect much more visible polar animals.

"I wouldn't like to speculate at the moment about whether it would affect penguins, whales and seals," he said. "At some stage it will. Whether that happens in the next 100 or 200 years is hard to predict."

Researchers have been worrying about both the north and south polar icecaps for almost a decade. The Arctic ocean ice shelf has thinned dramatically in the past 40 years and could disappear almost entirely by 2050. There is less evidence of any widescale warming on the Antarctic continent itself.

But the ice shelves of the Antarctic peninsula have proved far more fragile. Meteorologists have measured an average 2.5C warming there in the past 50 years. Since 1950, 13,500 sq kilometres (5,212 sq miles) of sea ice - enough to cover the island of Jamaica - have disintegrated. A further 3,400sq kilometres could disappear in the next decade.

There is still debate about whether the disappearing sea ice is part of a natural cycle or the first clear sign of global warming in the southernmost latitudes. Either way, the ice shelf is important in the Southern ocean ecosystem, but researchers are still not sure how such changes will affect the seabirds, seals and whales which feed and breed in the teem ing southern waters, but which also have large ocean ranges.

But some creatures vital to the ecosystem stay put - they have adapted to a very precise set of conditions, often living to great ages and growing to unprecedented sizes because of the higher levels of oxygen dissolved in near-freezing waters. Sea spiders in Antarctica have been weighed at 1,000 times heavier than they are in British waters, and are the size of a dinner plate. The sea gooseberry could grow to the size of a football, and isopods, a kind of aquatic woodlouse, could grow to 100 times the mass found off the British coast. These animals could reach such sizes because temperatures in the Southern ocean are constant. In some sites the temperature varied by one fifth of a degree in the course of a year, and had done so for 5m to 10m years, according to Prof Peck. The animals which had adapted to these constant temperatures died at rises of 4C to 10C in laboratory experiments.

Sea communities

"But even if before they get up to those experimental limits, they have physiological limits where they cannot get enough oxygen around the body, so they die in the long term, and those limits are about 3C to 6C," he said. "We have looked at swimming in Antarctic scallops; we have looked at the ability of limpets to turn themselves over - they lose those abilities at 2C to 3C."

Marine creatures were, paradoxically, more at risk from global warming than the tiny shrimps that lived in Antarctic freshwater lakes. These could survive a 50C temperature swing in the course of summer and winter. Their most pressing problem in a warmer world would be invasion from competitors. Sea communities, however, lived a finely balanced existence within a very precise temperature range.

But British and European climate scientists predict a sea temperature rise on average of about 2C in the next 100 years. This would take Antarctic ocean summer temperatures to exactly the point where these animals would feel the heat.

"So they will face problems from elevated temperatures in the next 100 years. Those temperatures are high in the sum mer, when the phytoplankton bloom and the food supply is there for them to exploit, but they might not be able to exploit it because of this limit to physiological capability," Prof Peck said.

"If the models are correct, and if the temperatures do get there, we are likely to lose at least large populations of these species - scallops, bivalve molluscs, the giant isopods, the giant sea spiders. In fact every species we have looked at so far falls into that category. We haven't found a species that does not.

"So it is not as if we looked at two or three. Every thing we looked at so far has problems even below 5C. It is not just a mite on some elk's nose dying somewhere. We've looked at herbivores, we've looked at detritivores, animals that are scavengers, and we've looked at carnivores. So it looks like it is all groups. For me the question is: is the sea going to change? Because if it does change in the way predicted, large-scale communities probably will suffer and we might have large-scale mortalities. It is probably the most fragile group of organisms on the planet in the face of a physical temperature change."

Antarctica attracts superlatives. It is the coldest, highest, windiest, driest place on earth

· Winter temperatures in the Antarctic have been recorded at minus 89.4C

· It is the highest continent, partly because of a cover of about 3,000 metres of packed snow and ice

· Much of the bedrock is below sea level, compressed by the weight of ice above

· It is the driest continent. One region, known as Dry Valleys, has had no rain for a million years

· It is the windiest continent. One cape is exposed to an average wind speed of 31mph (50kph)

· It is the emptiest, with a winter population of fewer than 300 people

· It is 58 times the area of Britain, twice the size of Australia, and the size of Canada, the continental US and Mexico combined

· Nine-tenths of all the world's fresh water sits on the land mass of Antarctica

· The area under ice doubles as 12m to 15m square kilometres (4.6m to 5.8m square miles) of sea ice forms each winter and melts again in summer